Abstract

Late Quaternary (c. 11,400 [sup]14 C years BP) palaeoclimates were reconstructed from the sediments of closed lake basins in the western (Laguna de Cuitzéo) and eastern (Laguna de Zempöala) trans-Mexican volcanic belt (TMVB). Climate was separated out as a distinct agent of environmental change from the influence of anthropogenic, tectonic and volcanic activity by the use of a range of complementary techniques. These included mineral magnetic, geochemical, ostracod species assemblages, organic carbon, C/N, and authigenic [delta][sup]18 O and [delta][sup]13 C analyses, the establishment of independent chronologies and synthesis with previous work.
At Laguna de Cuitzéo (core length, c. 11,400 BP) the Late Glacial was cold and wet with the Early Holocene (c. 1O,400-8,500 BP) arid. Wetter conditions established in the Mid Holocene (c. 7,500-1,300 BP) peaked with a period of methanogenesis (c. 6,600 BP). Drier conditions were recorded from c. 1,800 BP. An arid event c. 1,300 BP coincides with a peak in aridity identified in many records across the Sub-Tropics.
At Laguna de Zempöala (core length c. 3,000 BP) the Late Holocene was wet before c. 2,600 BP with a drier phase between c. 2,600-2,000 BP before the return of wetter conditions until c. 600 BP. A 'Mega-Drought' recorded between c. 600-300 BP, coincides with a historically documented event. Wetter conditions existed from c. 300 BP until present.
The high altitude closed lake basins of the TMVB provide high-resolution records of climate change. Mexico experienced a transition from a cold wet Late Glacial to an arid Early Holocene probably as a result of a northerly positioned Inter-Tropical Convergence Zone (ITCZ) displacing the north Atlantic subtropical high and weakening the easterly trade winds. Wetter conditions were established from c. 8,500 BP possibly because of orbital forcing extending the monsoon period. A transition occurred c. 1,800 BP into a period of apparent aridity (peaking c. 1,300-1,000 BP) probably resulting from solar forced variations, the periodicity of which modifies the intensity of the annual precipitation cycle leading to periods of drought and heavy rainfall on a bicentennial scale across the Sub-Tropics.